Papers of different densities have been provided for various purposes. Low density papers are soft and porous and have high absorbency unless treated to reduce absorbency. Medium density papers include papers utilized for writing, printing and wrapping purposes as well as bags and linerboards. Examples of high density papers include: glassine paper, grease-proof paper, vegetable parchment paper, vulcanized fiber paper and super-calendered paper.
An example of a dense writing paper is disclosed in my U.S. Pat. No. 3,839,144. Although that paper is dense and possesses excellent oil barrier properties, i.e. erasability, among other properties, it has certain limitations. For instance, the paper is not inexpensive to produce because it is manufactured from a furnish which is provided by heavily refining pulp to a predetermined Schopper-Riegler freeness and adding certain quantities of unrefined pulp thereto, or by combining alpha or cotton pulps with ordinary pulps and heavily refining the combined pulps, prior to the application of the furnish to the screen of a papermaking machine. It has been found that water drains relatively slowly from such heavily-refined furnishes so that it is very difficult to manufacture the desired dense paper at high machine speeds. Hence, it is more expensive to produce than other papers. Moreover, there is a limit on the maximum thickness of such a paper, and the necessity of heavily refining the pulp also increases the manufacturing cost of the paper.
Dense papers have certain desirable properties, including high tensile and burst (Mullen) strengths, improved folding endurance, improved interfiber bonding and delamination resistance, solvent and oil penetration resistance, and good abrasion resistance and rigidity. On the other hand, dense papers have low tear strengths, brittleness, poor dimensional stability and aging qualities and high manufacturing costs. For instance, vegetable parchment papers, and papers manufactured by the so-called vulcanized fiber processes, although dense, have relatively low tear strengths, as do dense papers manufactured by super-calendering webs of medium density.